Sendai virus is also named hemagglutinating virus of Japan (HVJ), and classified in parainfluenza virus type I, belonging to the genus Paramyxovirus of the family Paramyxoviridae.
Sendai virus particle is pleomorphic, having the genome RNA without a function as template for translation (hereafter designated “negative strand RNA”) enclosed in an envelope of 150-200 nm in diameter. Historically, Sendai virus has also been regarded as a biotechnologically useful virus, being widely utilized, especially for the production of heterokaryons and hybrid cells, by taking advantage of viral cell-fusion capacity. Also, Sendai virus-based cell fusing liposomes as a vehicle to deliver foreign genes into cells have been developed. Furthermore, Sendai virus is also used as the inducer for various interferons.
According to the classification based on the structure and polarity of genome nucleic acid, RNA viruses are classified into three groups, the double strand RNA viruses (dsRNA virus), positive strand RNA viruses, and negative strand RNA viruses. Sendai virus is a member of this third group (the negative strand RNA viruses). The dsRNA virus group includes reovirus, rotavirus, phytoreovirus, etc., and have segmented, multipartite filamentous dsRNA genomes. Positive strand RNA viruses include poliovirus, Sindbis virus, Semliki forest virus, and Japanese encephalitis virus, which possess a single molecule of positive sense RNA as genome. The genome RNA can function as an mRNA and is capable of producing proteins required for viral RNA replication and particle formation. In other words, the genome RNA itself of positive strand RNA viruses is infectious and capable of disseminating.
In the present specification, by “disseminative capability (spreading capability)” is meant “the capability to form infectious particles or their equivalent complexes and successively disseminate them to other cells following the transfer of nucleic acid into host cells by infection or artificial techniques and the intracellular replication of said nucleic acid. Sindbis virus classified in positive strand RNA viruses and Sendai virus classified in negative strand RNA viruses have both infectivity and disseminative capability. On the other hand, adeno-associated virus classified in the parvovirus family has the infectivity but no disseminative capability (the mixed infection of adenovirus is necessary for the formation of disseminating viral particles). Furthermore, the positive strand RNA derived from Sindbis virus which is artificially transcribed in vitro is disseminative (to form infectious viral particles when transfected into cells). In contrast, not only genomic negative strand but also antigenomic positive strand of Sendai viral RNA artificially transcribed in vitro cannot serve as a functional template to form infectious viral particles when transfected into cells.
Recently, viral vectors have been used as vehicles for gene therapy. In order to use them as gene therapy vectors, it is necessary to establish techniques for reconstituting viral particles. (By “reconstitution of viral particles” is meant the artificial formation of viral genome nucleic acid and the production of original or recombinant viruses in vitro or intracellularly.) This is because, in order to transfer foreign genes into viral vectors, viral particles should be reconstituted from the viral genome with foreign genes integrated by gene manipulation. Once techniques of viral reconstitution are established, it becomes possible to produce viruses with a desired foreign gene introduced, or with desired viral genes deleted or inactivated.
Also, once the viral reconstitution system is constructed and the viral gene manipulation becomes possible, said system appears to become a potential tool for genetically analyzing the viral function. Genetic analysis of viral functions is very important from the medical viewpoint of prevention and therapy of diseases etc. For example, if the replication mechanism of viral nucleic acid is elucidated, by utilizing the differences between said viral metabolism and host-cellular metabolism, it may be possible to develop viricide acting on the viral nucleic replication process and less damaging to host cells. Also, by elucidating functions of viral gene-encoded proteins, it may become possible to develop antiviral drugs targeting proteins related with the viral infectivity and particle formation. Furthermore, by modifying genes concerned with the membrane fusion and preparing liposomes with superior membrane-fusing capability, it will be able to use them as gene therapy vectors. In addition, as represented by the interferon, the viral infection may induce the activation of host genes for viral resistance, resulting in the enhanced viral resistance of hosts. Genetic analysis of virus functions may provide more important information on the activation of host genes.
Reconstitution of DNA viruses possessing DNA as the genomic nucleic acid has been performed for some time, and can be carried out by the introduction of the purified genome itself, such as SV40, into monkey cells [J. Exp. Cell Res., 43, 415-425 (1983)]. Reconstitution of RNA viruses containing an RNA genome has been preceded by positive strand RNA viruses due to the dual function of these genomes as mRNA and the template for replication. For example, in the case of poliovirus, the disseminative capability of the purified genomic RNA itself was already demonstrated in 1959 [Journal of Experimental Medicine, 110, 65-89 (1959)]. Then, it was achieved to reconstitute poliovirus [Science, 214, 916-918 (1981)] and Semliki forest virus (SFV) [Journal of Virology, 65, 4107-4113 (1991)] by the introduction of cloned cDNAs into host cells, which encoded the respective full-length plus strand viral RNAs.
The infectious cycle begins with the viral RNA synthesis from DNA, catalyzed by cellular DNA-dependent RNA polymerase. Furthermore, using these viral reconstitution techniques, gene therapy vectors have been developed [Bio/Technology, 11, 916-920 (1993); Nucleic Acids Research, 23, 1495-1501 (1995); Human Gene Therapy, 6, 1161-1167 (1995); Methods in Cell Biology, 43, 43-53 (1994); Methods in Cell Biology, 43, 55-78 (1994)].
However, as described above, in spite of many advantages of Sendai virus to be biotechnologically and industrially useful virus, its reconstitution system has not been established, because it is a negative-strand RNA. This is due to tremendous difficulty in reconstituting viral particles via viral cloned cDNA because neither genomic nor antigenomic RNA alone expressed from the cDNAs is active as the templates for mRNA synthesis and genome replication. This is absolutely different from the case of positive strand RNA viruses. Although, in JP-A-Hei 4-211377, “methods for preparing cDNAs corresponding to negative strand RNA viral genome and infectious negative strand RNA virus” are disclosed, the entire experiments of said documents described in “EMBO. J., 9, 379-384 (1990) were later found to be not reproducible, so that the authors themselves had to withdraw all the article contents [see EMBO J., 10, 3558 (1991)]. Therefore, it is obvious that techniques described in JP-A-Hei 4-211377 do not correspond to the related art of the present invention. Reconstitution systems of negative strand RNA viruses were reported for influenza virus [Annu. Rev. Microbiol., 47, 765-790 (1993); Curr. Opin. Genet. Dev., 2, 77-81 (1992)]. Influenza virus is a negative strand RNA virus having eight-segmented genomes. According to these literatures, a foreign gene was first inserted into the cDNA of one of said genome segments, and then RNA transcribed from the cDNA containing the foreign gene was assembled with the virus-derived NP protein to form a ribonucleoprotein complex (RNP). Then, cells are transfected with the RNP and further infected with an intact influenza virus, in which the corresponding gene segment does not function under special pressure (such as the presence of neutralizing antibody and high temperature). In cells, gene reassortment occurs to generate a virus in which the genome segment is replaced with the above engineered segment to contain a foreign gene in a small population. This population is then selected and amplified under the pressure described above, thus ultimately generating a desired recombinant virus. Thereafter, the reconstitution of a nonsegmented negative strand RNA virus entirely from cDNA was reported for rabies virus belonging to the rhabdovirus family [EMBO J., 13, 4195-4202 (1994)].
Therefore, techniques for reconstituting negative strand viruses have become fundamentally known to the public. However, Sendai virus belongs to the Paramyxovirus family, different from the Rhabdovirus family. Sendai virus and rabies virus could differ in detailed mechanisms of gene expression and replication. They also differ in protein components and virrion structure. Probably, for their reasons, the direct application of the above-described techniques for rabies virus did not support Sendai virus reconstitution. Also, the reconstitution of viral particles reported on the rhabdovirus was hardly detectable by routine virological procedures such as plaque production on susceptible cell cultures. Furthermore, the yield was not satisfactorily high for practical applications. Besides, in order to provide factors required for the viral reconstitution within host cells, helper viruses such as wild type viruses, recombinant vaccinia virus, etc. were conventionally introduced to host cells together with nucleic acids of the virus to be reconstituted. Accordingly, difficulties in separating the reconstituted desired virus from these harmful viruses were posing a difficult problem.